Mushrooms are grown commercially in the United States and elsewhere in great volumes to satisfy the culinary demand by consumers for this edible commodity. Many varieties are grown, with the well-known white/off-white mushrooms by far the most common, and more exotic varieties such as shitaki, enoki, and the like produced in lesser quantities. These fungi all present significant challenges to the grower who must duplicate the growing conditions found in nature for each variety while optimizing his production costs and efficiencies.
White mushroom fanning, as representative of mushroom production, consists of six steps: Phase I composting and Phase II composting followed by spawning, casing, pinning, and cropping, in succession. Composting involves preparation of the nutrient base (Phase I) and pasteurization/de-ammonifization (Phase II) for the mushrooms. See, Wuest, Duffy, and Royce, Six Steps to Mushroom Fanning, Penn State Univ., Col of Ag. Sci.--Coop. Ext. Spec. Cir. 268 circa 1979, incorporated herein in its entirety. Spawning is the process by which the grower inoculates the compost with the mushroom "spawn", (mushroom mycelia propagated vegetatively).
In Step 4, the spawn-ran compost is cased, whereby a top-dressing of selected materials (typically, clay-loam field soil, a mixture of peat moss with ground limestone, or reclaimed, spent compost) is spread uniformly over the surface of the compost on which the mushrooms eventually form. This casing is typically pre-wet to a high moisture level, and thereafter acts as a water reservoir and a place for the growth and fusion of mycelia into rhizomorphs. Without rhizomorphs, no primordia, or pins, form, and there would be no mushrooms. Uniformity of the casing over the compost is very important because it allows the spawn to move into and through the casing at the same rate. Additionally, it is critical that the casing medium be able to hold water, as the continuous availability of moisture is essential for the development of a firm, marketable mushroom of acceptable size, and, ultimately for profitable yields. Throughout the period following casing, water is applied intermittently to maintain the moisture level. Knowing when, how, and how much water to apply to the casing material is considered an art form in the industry and critical to efficient production.
Mushroom initials develop as outgrowths on rhizomorphs formed in the casing. The initials grow in size to form structures referred to as pins, which in turn continue to expand and grow through a button stage and ultimately enlarge into a mushroom. Depending upon growing conditions, mushrooms can be harvested 18-21 days after casing. Pin development can be controlled, in part, by the concentration of carbon dioxide in the atmosphere above the casing. Optimal pin development is dependent upon a time reduction of carbon dioxide concentration, along with maintenance of sufficient moisture and relative humidity. Buttons continue to develop and enlarge through the cropping period. Individual crops or "breaks" are gathered during repeating 3-5 day harvests throughout the cropping phase. Several breaks may be harvested in succession followed by a several day period in which no new mushrooms appear. This break/harvest cycle is repeated several times during cropping, which may last anywhere from 35-150 days depending on the mushroom variety and growing technique.
While each phase or step in the mushroom production process is critical to the growth cycle and the overall yields obtained, the casing Step 4 presents particular problems and the opportunity for unique solutions. One approach, used with limited success is to add the so-called "superabsorbant" polymers to the casing medium to increase moisture availability to the mushroom spawn. None of these additives have met with any degree of commercial success, due to a number of significant problems and deficiencies. First of all, the superabsorbants are highly cross-linked polymers which form gel networks, absorbing many times their weight in water. However, due to their high gel strength the superabsorbants (hydrogels) do not readily give up their water to the growing mushroom mycelia. Second, because these superabsorbant polymers are gels and water-insoluble under use conditions, they present a discontinuous lump, or water reservoir, which is only available to mycelium in the immediate vicinity of the gel network. As a result, they do not coat the peat strands. Third, at the concentrations used in mushroom production (1-3% by weight), they do not form a supply of water sufficient for the growing mushrooms and, in fact, compete with the growing mushroom mycelium for the available water supply. Fourth, they are difficult to add to the casing mixture because of their tendency to agglomerate and clump. The superabsorbants do not wet out on a peat strand and are difficult to uniformly mix throughout the casing. Finally, additives of this sort are expensive, averaging from $2.50 to $6.00 per pound, and available only at costs which unduly cut profit margins and render them unfeasible for widespread use.
In summa, a considerable number of drawbacks and problems exist in the art relating to synthetic polymers for use as additives to casings in mushroom production. Standard cultivation practices define a need for a casing material and/or an amendment which facilitates the ability of the mycelia to move therethrough and maximizes access to the available water supply.